When transmitting radio signals, a transmitter for the radio signal usually comprises a power amplifier. The power amplifier can be operated in a number of different modes of operation, such as a class A amplifier, a class B amplifier, a class AB amplifier, a class C amplifier, and a class D amplifier. The choice of one of these classes as the mode of operation for the power amplifier is usually a compromise between signal distortion and power efficiency. For example, a class A amplifier offers a very low degree of signal distortion, but has poor power efficiency. A class D amplifier, on the other hand, exhibits a very good power efficiency but distorts the input signal to a sequence of digital pulses having the same amplitude.
In the domain of wireless mobile communications, such as used in cellular communication networks, another factor comes into play: the spectrum useable for wireless communications is a limited resource which needs to be exploited efficiently with the increasing demand for wireless communication and applications. This need for efficient spectrum utilization was one of the driving forces for the development of new standards for wireless communication, such as the family of standards for mobile communications commonly termed “third generation”, or 3G. The 3G family includes UMTS (Universal Mobile Communication System), CDMA 2000 (Code Division Multiple Access), DECT (Digital Enhanced Cordless Telecommunications), and WiMAX (World wide Interoperability for Micro wave Access) standards, among others. These third generation standards, while offering a more efficient utilization of spectrum resources than earlier the older wireless communications standards, make heavy demands on the linearity of the equipment used to process the signal, i.e. transmitters and receivers. For the transmitter this typically means that the power amplifier needs to operate in the linear region.
The linear region of the power amplifier has to be large enough to receive the dynamic range of the telecommunications signal to be amplified. A power amplifier with a large linear range of operation typically is more expensive and consumes more power than a power amplifier with a smaller linear range of operation. The required size of the linear range is, among others, determined by a property of an input signal called “crest factor”. The crest factor is the ratio between a maximum peak and an average value of a signal. Faced with a signal with a high crest factor the power amplifier needs to be designed for the maximum peak value, even though the maximum peak value only occurs very scarcely.
The crest factor reduction of digital radio signals is desirable in third generation cellular network base station radio front end equipment in order to achieve high power amplifier efficiency. Generally, the higher the crest factor, the more back-off is necessary when designing an analog power amplifier. A high back-off results in a reduced efficiency for most state-of-the art power amplifier designs.
For a standard transmitter, such as in a remote radio head (RRH), the crest factor reduction may be applied directly to the multi-carrier signal prior to feeding the multi-carrier signal into the power amplifier.
Several state-of-the art approaches and reference designs exist for reducing the crest factor of the multi-carrier signals. An international patent application published under the international publication number WO 2004/019540 A2 discloses a crest factor reduction processor for wireless communications. A plurality of peak detection and cancellation circuits is arranged in a sequence in the WO'540 application. This serves to reduce peaks that, as a result of “peak regrowth”, are caused at sample points near to a reduced peak point.
U.S. Pat. No. 7,313,373 B1 discloses a crest factor reduction for use in a multi-band transmitter capable of transmitting a plurality of component signals that are associated with dedicated sub-bands. The component signals are superposed and the superposed signal is processed to form a clipping noise error signal. The clipping noise error signal is applied to the component signals using a least square estimation to project clipping noise error onto the sub-bands.
A number of scientific papers by Wan-Jong Kim also address crest factor reduction techniques. These articles are:    IEEE Microwave and wireless components letters, vol. 17, no. 1, January 2007: “Doherty feed-forward amplifier performance using a novel crest factor reduction technique”    Analog Integrated Circuits and Signal Processing (2007) 150: 19-26: “An efficient crest factor reduction technique for wide band applications”    Thesis (Ph.D)—School of Engineering Science, Simon Fraser University, fall 2006: “Digital pre-distortion linearization and crest factor reduction for wide band applications”.
An application note by Xilinx, Inc. entitled “Peak cancellation crest factor reduction reference design”, identification no. XAPP 1033 (v 1.0 Dec. 5, 2007), describes a peak cancellation method based on a generation of a cancellation pulses. The peak cancellation is achieved by subtracting spectrally shaped pulses from the signal peaks that exceed a specified threshold. The cancellation pulses are designed to have a spectrum that matches that of the high-crest factor input signal and therefore introduce only negligible out-of-band interference. For multi-carrier configurations, the Xilinx application note proposes the creation of a composite multi-band filter. Each of the cancellation pulses is filtered by the composite multi-band filter and accordingly occupies portions of the spectrum that correspond to the transmission band(s) of the multi-band filter. In active antennas with a distributed transceiver architecture, the CFR arrangement proposed by Xilinx would have to be implemented in each transmitter path because that is where the combining of the single-carrier signals to the multi-band signal occurs.